Continuous measurement of the thickness of hot thin films

ABSTRACT

Described herein is a method for monitoring the thickness of a thin transparent film while it is growing on a hot substrate by vapor deposition. In a specific example, two silicon substrates, of which one has a predeposited coating of silicon dioxide, are located close together to be subjected to a similar vapor deposition of alumina films. The difference in electromagnetic radiation emitted by each substrate and transmitted through each film is monitored, and the deposition process is terminated just after this difference reaches a prescribed value.

I United States Patent l 13,620,814

[72] Inventors Conrad A. Clark [56] Relerences Cited gzg li) b E t J FUNITED STATES PATENTS n urn n, as on; antes Roberts, Bethlehemallot pa.3,099,579 7/1963 Sp1tzer et a1. 117/106 X [21] Appl. No. 751,597 PrimaryExaminer-Alfred L. Leavitt {22] Filed Aug. 9, 1968 AssistantExaminer-Wm. E. Ball [45] Patented Nov. 16, 1971 Attorneys-R. J.Guenther and Arthur J. Torsiglieri [73] Assignee Bell TelephoneLaboratories, incorporated Murray Hill, Berkeley Heights, NJ. v

ABSTRACT: Described herein is a method for monitoring the [54]CONTINUOUS MEASUREMENT OF THE tliiglinetsstof thin :jransparent filmwhile it Is growing on a hot THICKNESS or nor THIN FILMS S s y 4 c 3 DIn a specific example, two silicon substrates, of which one has rawmg apredeposited coating of silicon dioxide, are located close [52] US. Cl117/106 R, together to be subjected to a similar vapor deposition ofalu- 356/108, l17/DlG. 2 mina films. The difference in electromagneticradiation [51] Int. Cl C23c 11/00 emitted by each substrate andtransmitted through each film is Field of Search 117/ 106, monitored,and the deposition process is terminated just after DIG. 2;356/51, 108,161 168, 204, 206, 229

this difference reaches a prescribed value.

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E E MT T OM M w w W .B U U a s N oA 0 0% m U W 2nd 5 2 E R A B m IN A OFAL2O3 DEPOSITED THICKNESS ON SILICON SUBSTRATE C. A. CLARK lNl/ENTOHS A.C. DUMBR/ J. F. ROBERTS [3V ATTOPNFY PATENTED 16197! 3.620.814

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3a, PHOTOMULTIPLIER ROTATlNG SEAL 30, BELL JAR CONTINUOUS MEASUREMENT OFTHE THICKNESS OF HOT THIN FILMS BACKGROUND OF THE INVENTION Thisinvention relates to a method for determining the thickness of a growingtransparent thin film as it is being deposited upon a heated substrate,in particular, a semiconductive substrate.

In the growth of thin films, especially by vapor deposition, the rate ofdeposition is difficult to reproduce; thus it is desirable to have amethod for monitoring the thickness of the film as it is growing in situby vapor deposition. It should be understood that the term vapordeposition is used broadly herein, and includes (but is not limited to)such processes as vacuum evaporation, pyrolitic decomposition, plasmadeposition, and sputtering.

Of course, several methods exist in the art for measuring the filmthickness after the film has been deposited and the deposition processterminated. See, for example, Thin Film Microelectronics, L. Holland(Editor), 1965.

Also, methods are known in the prior art (see for example U.S. Pat. No.3,099,579, issued July 30, l963) for determining in situ the thicknessof a transparent thin film growing on a hot substrate, by monitoringexternally supplied electromagnetic radiation reflected and transmittedtherethrough; but such methods suffer from the disadvantage that theradiation emitted by the hot substrate itself obfuscates thisdetermination. Likewise, when the film is being deposited by vapordeposition techniques, accumulations of deposits on the window of thedeposition chamber further confuses the film thickness determination insitu. Thus, it is desirable to have a method for determining thethickness of such thin films while they are growing in situ, whichavoids these complicating disadvantages.

SUMMARY OF THE INVENTION Continuous measurement of the thickness of agrowing transparent, or at least semitransparent, thin film upon a hotsubstrate is accomplished by monitoring the intensity of theelectromagnetic radiation in a given, preferably narrow, spectral range.This radiation is that which is emitted by the hot substrate andtransmitted through the film.

As is known from the theory of optical interference, the transmissivityof a nonconducting thin film whose refractive index is lower than thatof a nonconducting emitting substrate, yields minima and maximaapproximately in accordance with the formulas nk=2t(minimal) (l)(minima) and (n+'r)h=2t(maxima) (2) where n is an integer or zero; A isthe wavelength measured in the film of the radiation being monitored;and t is the thickness of the film. As previously stated, Equations (1)and (2) hold, provided the refractive index of the film is lower thanthat of the emitting substrate; otherwise equation (1) applies to maximaand equation (2) to minima in case the refractive index of the film ishigher than that of the emitting substrate.

In any case, if either the thin film or the substrate is electricallyconducting, then equations (1) and (2) do not hold, due to phase shiftsdifferent from to -:r upon reflection and transmission at interfaces.However, maxima and minima do occur in any event, although theirrelationship with thickness of the film may contain an additional termrepresenting these phase shifts, as compared with equations (1) and (2).Thus, as the film grows in thickness, whether or not the substrate orthe film is conductive, the intensity of the radiation of a givenwavelength emitted by the hot substrate, and as transmitted through thefilm, undergoes an oscillatory behavior between maxima and minima. Byterminating the deposition process immediately after a predeterminednumber (integral or fractional) of oscillations have occurred, apredetermined desired thickness of film is thereby deposited.

Furthermore, in order to avoid complications caused by fluctuations oftemperature of the substrate, as well as those caused by the depositionof film upon the windows of the deposition chamber, it is advantageousto proceed with two substrates, one of which has a predeposit of anothertransparent, or at least semitransparent, coating of preselectedthickness. Except for the coating, the two substrates are otherwiseidentical; but this is not essential. The material used in thispredeposited coating is desirably selected from among those which areknown to be relatively easy to grow reproducibly to a preselectedthickness upon the substrate. The two substrates, only one of which hasthe aforesaid predeposited coating, are then placed in mutually closeproximity (for thermal equilibrium between them) while the depositionprocess of the desired film is carried out at the same rate upon bothsubstrates at the same elevated temperature. While this deposition istaking place, the electromagnetic radiation emitted by the substratesand transmitted through the films is either periodically or continuouslymeasured, within a given spectral range, i.e., wavelength interval. Thedifference between the two intensities of radiation thus monitored fromthe two substrates will itself have an oscillatory behavior with time.Thus a film of the desired thickness will be deposited (upon bothsubstrates) immediately after a predetermined (integral or fractional)number of such oscillations have taken place. The calibration of thenumber of such oscillations with respect to film thickness may be madeby earlier experiment, or by utilizing the fact that each suchoscillation corresponds to an increase in thickness of one half theaverage wavelength of monitored radiation, said average wavelength asmeasured in the film however. Advantageously, the intensity ofelectromagnetic radiation emitted from each substrate is monitored by aseparate photodetector; and their output ratio for given incidentradiation is adjusted so that the difference in their outputs is zerojust when the desired film thickness is reached, so that more accuratecontrol over the resulting thickness is obtained. This inventiontogether with its advantages, features, and objects may be betterunderstood from the following detailed description when read inconjunction with the drawing in which:

FIG. I is a diagram, not to scale, of apparatus useful for carrying outa specific embodiment of this invention;

FIG. 2 is a plot of the radiation intensity versus film thickness,obtained in a specific embodiment of this invention; and

FIG. 3 is a diagram, not to scale, of application useful for carryingout another specific embodiment of this invention.

EXAMPLE I Referring to FIG. 1, silicon substrates 1 and 1A are locatedinside a deposition chamber 2 having a window 3 transparent, or at leastsemitranslucent, to the radiation from the substrates 1 and 1A to bemonitored. Lenses 4 and 4A collect and focus this radiation fromsubstrates 1 and IA respectively upon the surfaces of photodetectors 5and 5A, chosen to be sensitive to the radiation. Stops, not shown,prevent radiation from substrate 1 from reaching photodetector 5A, andprevent radiation from 1A from reaching 5. The outputs 6 and 6A of thesephotodetectors are fed into the difference amplifier 7, whose output ismonitored by means (not shown) familiar to those skilled in the art.

The substrate 1A has a predeposit of a coating 8 of SiO, (silicondioxide) of prescribed thickness, 810 A thick for example. This coating8 may be obtained by methods known in the art, such as thermal growth insteam. The prescribed thickness may be obtained by predetermining thedeposition rate by methods known in the art, such as either visible orultraviolet spectrophotometry, or else by multiple beam interferometry.Both the substrates 1 and 1A are subjected to the same vapor depositionprocess, as known in the art, of a desired transparent film material, A10 (alumina) for example. The provision of a thin layer of alumina orsilicon is useful in the fabrication of one form of insulated gatefield-effect transistor (IG- FET). The substrates 1 and 1A are locatedclose together so that the films 9 and 9A grow at the same rate and thesubstrates are in thermal equilibrium with each other. The substrates 1and 1A are maintained typically at a temperature of 900 C. or moreduring the deposition process, so that they radiate appreciable amountsof spectral radiation in the neighborhood of 6,000 A vacuum wavelength.While the deposition proceeds, layers of the A1 films 9 and 9A grow uponthe substrates 1 and 1A respectively, and the difference between theoutputs 6 and 6A of the photodetectors 5 and 5A respectively is measuredby the difference amplifier 7. When When this difference reaches aprescribed value after a prescribed number of oscillations, the vapordeposition process is terminated. At this point, both substrates 1 and1A will have a film 9 and 9A of the desired thickness, as will moreclearly appear from the following discussion of FIG. 2. Advantageouslyfor optimum accuracy, the parameters are adjusted so that the desiredfilm thickness is reached when the difference between the outputs 6 and6A is equal to zero, as will also more clearly appear from thefollowing.

For sharper results, filters (l0 and A) known in the art are placedbefore the photodetectors 5 and 5A. These filters are selected totransmit only a narrow range (30A half bandwidth) of the spectrum of theradiation. Thus, the photodetectors measure the intensity of radiationin this narrow spectral range emitted by the hot substrates 1A and 1, astransmitte through the layers 8, 9A, and 9.

Referring to FIG. 2, curve 21 shows a calibration plot of the intensityof radiation centered at 6,000 A (vacuum wavelength) versus thickness ofA1 0 film 9 being deposited on substrate 1 at 925 C. This curve 21 isobtained by means of measuring the film thickness subsequent to variousdepositions, by methods known in the art. It should be noted that thedistance along the horizontal axis between the first maximum and thefirst minimum (i.e., one half an oscillation) is 860 A. Thus, a completeoscillation in the curve 21 in FIG. 2 would correspond to a deposit ofA1 0, whose thickness is equal to l,720 A, i.e., one-half the wavelengthin the film 9 (of A1 0 at 925 C.) of the radiation having a vacuumwavelength of 6,000 A. Hence, oscillations or fractions thereof in curve21 is a measure of thickness of the A1 0 film 9 on the substrate 1.

Curve 22 in FIG. 2 shows a plot of the intensity of radiation, centeredat 6,000 A vacuum wavelength, versus thickness of the A1 0 film 9Agrowing on the 810 A thick SiO, coating 8 on the silicon substrate 1A.Due to the predeposited SiO, coating 8, the curves 22 and 21 are out ofphase, that is, their maxima and minima do not correspond to the same A10; film thickness. It should be remembered that due to the mutuallyclose positioning of the substrates 1 and 1A, the thicknesses of films 9and 9A are essentially equal to each other at all times during theirgrowth. Curves 21 and 22 intersect, for the first time starting fromzero thickness, at the point 23; corresponding to an A1 0 film thicknessof 470 A for both films 9 and 9A. Thus, for a desired thickness of A1 0films 9 and 9A equal to 470 A, the deposition process is terminated onthe first occasion at which the difference in the outputs 6 and 6A,measured by the difference amplifier 7, gives a null. Since nulls areeasier to detect than other quantities, it is advantageous to utilizesuch nulls in the determinations of the thicknesses of films 9 and 9A.

The degree to which the curves 21 and 22 are out of phase depends uponthe original thickness of the SiO coating 8 on the substrate 1A, amongother things. Hence, by choosing this original SiO coating 8 to bedifferent from the 810 A assumed above, the first null, i.e., thecrossing point 23 of the curves 21 and 22 can be made to occur at aposition corresponding to a thickness of A1 0 films 9 and 9A differentfrom the above 470 A. Likewise, by waiting until the second (later)crossing (off scale in FIG. 2, at l,4l0 A thickness of A1,0,) produces asecond null in the output of the difference amplifier 7, a thickness ofA1 0 films 9 and 9A would be obtained equal to 1,410 A thick.

The crossing point 23 may also be made to correspond to a differentvalue of thickness of films 9 and 9A from the above 470 A, by varyingthe ratio of the apertures of the photodetectors 5 to 5A, or by varyingthe ratio of their outputs 6 and 6A for given incoming radiationintensity, as known in the art. Such adjustments of the photodetectorshave the effect of vertically displacing curves 21 and 22 with respectto each other, thereby changing the abscissa of their crossing point 23,as desired.

EXAMPLE II Referring to FIG. 3, a plurality of substantially identicalclean silicon slice substrates 310, b, c...are located inside a quartzbell jar 30 on a pedestal 32. For definiteness, substrate 31a is calledthe control" substrate. Silicon slice substrate 31A has a predepositedcoating of silicon dioxide of prescribed thickness, 810 A thick forexample as described in example I above. The pedestal 32 together withall the substrates are all heated by means of radio frequency heatingcoil 33. The pedestal 32, mounted on a shaft 34, is rotated by the beltdrive 35A driven by the motor 35. A rotating seal 36 is provided alongthe periphery of the base 37, to seal the bell jar 30.

The vapor, containing the aluminum oxide to be deposited, is introducedinto the bell jar 30 through the gas inlet 36. As the shaft 34 rotatesthe pedestal 32, the photomultiplier head 38 detects the electromagneticradiation emitted by that substrate on the pedestal 32 which isinstantaneously aligned. A cam 39 attached to the shaft 34 engages eachof the microswitches 41 or 42 once per rotation of the shaft. Inparticular, when the precoated substrate 31A is instantaneously alignedwith the photomultiplier head 38, the microswitch 41 is positioned to beinstantaneously closed temporarily, but it is otherwise open. Likewise,when the control" substrate 31A is aligned with photomultiplier head 38,the cam 39 temporarily closes the microswitch 42. The substrates 31A and31a are thus located on the pedestal 32 in relation to the cam 39 inpredetermined positions to accomplish these closings of themicroswitches 41 and 42, to which the output of the photomultiplier 38is connected electrically.

Microswitches 41 and 42 are electrically connected respectively toamplifiers S1 and 52, respectively. Each of these amplifiers is in acathode follower negative voltage feedback arrangement, to present aninput impedance which is much larger than the impedance of thephotomultiplier 38 when operating. Capacitors C and C are connected toamplifiers 51 and 52 respectively, to furnish a time constant which islarge compared with the period of rotation of the pedestal 32. The inputvoltage signals e, and e, to the amplifiers 51 and 52, respectively, aresupplied by the photomultiplier 38 in response to the electromagneticradiation emitted by the substrates 31A and 31a. The output signals ofthe amplifiers 51 and 52 both are thus constant signals for one rotationof the pedestal 32, and may change only stepwise in response to thefresh signal supplied. from the photomultiplier upon each new rotation(during the closing of the microswitches 41 and 42), in response to theradiation from the substrate 31A or 31a. These output signals ofamplifiers 51 and 52 are supplied to the difference amplifier 53,through resistors R and R Negative feedback resistor R, is advantageousin limiting the gain of amplifier 53 to a desirable range fortransmission to the detector 54. Resistor R, stabilizes the amplifier53, if desired.

Resistor R is adjustable, in order to control the relative sensitivityof the difference amplifier 53 to the two signals from amplifiers 51 and52, to correct for such imbalances as the inequality of gain inamplifier 51 versus 52, for example. Thus, the detector 54 efi'ectivelypresents the difference between the radiation being emitted in the belljar 30 by substrates 31A and 310 as transmitted through the films beingdeposited thereon as the deposition process proceeds. This differenceundergoes an oscillatory behavior as the deposition process builds upthe films upon the substrates. When this difference reaches apredetermined value, advantageously zero, after a predetermined numberof oscillations, the deposition process is terminated immediately. Atthis time, the substrates 31a, b, c...will all have a film of aluminumoxide of predetermined desired thickness, as should be obvious fromexample I above.

It should now be obvious to those skilled in the art how to modify theabove detailed examples to apply the same technique to other parameterssuch as other substrates, substrate temperatures, predeposited coatings,and transparent films desired of given thickness. Likewise, depositionchambers such as described in copending application, D. R. Oswald Ser.No. 74744,4l5, filed Julyi2, 1968 and having the same assignee as thepresent application, may also be used in the practice of this invention.

Although this invention has been set forth in terms of detailedexamples, various modifications are possible within the scope of theinvention.

What is claimed is:

1. (Twice amended) The method of growing, upon a first siliconsubstrate, an aluminum oxide film having a desired thickness comprisingthe steps of:

a. forming a silicon dioxide coating of preselected thickness upon asecond silicon substrate, said coating being at least semitransparent ina predetermined spectral range of the electromagnetic radiation emittedby the second substrate at an elevated temperature;

b. placing said first and said second substrates in a heated environmentat the elevated temperature, so that both the first and the secondsubstrates are heated to the same temperature;

c. growing the aluminum oxide film upon both said substrates at equalrates by vapor deposition; and

d. monitoring the difference in the intensities of the electromagneticradiation emitted by both said substrates ad transmitted through thefilms thereon in the predetermined spectral range.

2. The method of claim 1 in which the growing is terminated when thedifference in intensities is zero.

3. The method of claim 1 in which the spectral range is centered atabout 6,000 A, and the elevated temperature is of the order of 900 C.

4. The method of claim 1 in which the thickness of the silicon dioxidecoating is about 810 A.

2. The method of claim 1 in which the growing is terminated when thedifference in intensities is zero.
 3. The method of claim 1 in which thespectral range is centered at about 6,000 A, and the elevatedtemperature is of the order of 900* C.
 4. The method of claim 1 in whichthe thickness of the silicon dioxide coating is about 810 A.